1. Field of the Invention
This invention relates to computer system data storage, and more particularly to a system for generating redundancy information in each redundancy data storage unit within a redundant array system.
2. Description of Related Art
A typical data processing system generally involves one or more storage units which are connected to a Central Processor Unit (CPU) either directly or through a control unit and a channel. The function of the storage units is to store data and programs which the CPU uses in performing particular data processing tasks.
Various types of storage units are used in current data processing systems. A typical system may include one or more large capacity tape units and/or disk drives (magnetic, optical, or semiconductor) connected to the system through respective control units for storing data.
However, a problem exists if one of the large capacity storage units fails such that information contained in that unit is no longer available to the system. Generally, such a failure will shut down the entire computer system.
The prior art has suggested several ways of solving the problem of providing reliable data storage. In systems where records are relatively small, it is possible to use error correcting codes which generate ECC syndrome bits that are appended to each data record within a storage unit. With such codes, it is possible to correct a small amount of data that may be read erroneously. However, such codes are generally not suitable for correcting or recreating long records which are in error, and provide no remedy at all if a complete storage unit fails. Therefore, a need exists for providing data reliability external to individual storage units. Other approaches to such "external" reliability have been described in the art. A research group at the University of California, Berkeley, in a paper entitled "A Case for Redundant Arrays of Inexpensive Disks (RAID)", Patterson, et al., Proc. ACM SIGMOD, June 1988, has catalogued a number of different approaches for providing such reliability when using disk drives as failure independent storage units. Arrays of disk drives are characterized in one of five architectures, under the acronym "RAID" (for Redundant Arrays of Inexpensive Disks).
A RAID 1 architecture involves providing a duplicate set of "mirror" storage units and keeping a duplicate copy of all data on each pair of storage units. While such a solution solves the reliability problem, it doubles the cost of storage. A number of implementations of RAID 1 architectures have been made, in particular by Tandem Corporation.
A RAID 2 architecture stores each bit of each word of data, plus Error Detection and Correction (EDC) bits for each word, on separate disk drives. For example, U.S. Pat. No. 4,722,085 to Flora et al. discloses a disk drive memory using a plurality of relatively small, independently operating disk subsystems to function as a large, high capacity disk drive having an unusually high fault tolerance and a very high data transfer bandwidth. A data organizer adds 7 EDC bits (determined using the well-known Hamming code) to each 32-bit data word to provide error detection and error correction capability. The resultant 39-bit word is Written, one bit per disk drive, on to 39 disk drives. If one of the 39 disk drives fails, the remaining 38 bits of each stored 39-bit word can be used to reconstruct each 32-bit data word on a word-by-word basis as each data word is read from the disk drives, thereby obtaining fault tolerance.
An obvious drawback of such a system is the large number of disk drives required for a minimum system (since most large computers use a 32-bit word), and the relatively high ratio of drives required to store the EDC bits (7 drives out of 39). A further limitation of a RAID 2 disk drive memory system is that the individual disk actuators are operated in unison to write each data block, the bits of which are distributed over all of the disk drives. This arrangement has a high data transfer bandwidth, since each individual disk transfers part of a block of data, the net effect being that the entire block is available to the computer system much faster than if a single drive were accessing the block. This is advantageous for large data blocks. However, this arrangement effectively provides only a single read/write head actuator for the entire storage unit. This adversely affects the random access performance of the drive array when data files are small, since only one data file at a time can be accessed by the "single" actuator. Thus, RAID 2 systems are generally not considered to be suitable for computer systems designed for On-Line Transaction Processing (OLTP), such as in banking, financial, and reservation systems, where a large number of random accesses to many small data files comprises the bulk of data storage and transfer operations.
A RAID 3 architecture is based on the concept that each disk drive storage unit has internal means for detecting a fault or data error. Therefore, it is not necessary to store extra information to detect the location of an error; a simpler form of parity-based error correction can thus be used. In this approach, the contents of all storage units subject to failure are "Exclusive OR'd" (XOR'd) to generate parity information. The resulting parity information is stored in a single redundant storage unit. If a storage unit fails, the data on that unit can be reconstructed onto a replacement storage unit by XOR'ing the data from the remaining storage units with the parity information. Such an arrangement has the advantage over the mirrored disk RAID 1 architecture in that only one additional storage unit is required for "N" storage units. A further aspect of the RAID 3 architecture is that the disk drives are operated in a coupled manner, similar to a RAID 2 system, and a single disk drive is designated as the parity unit.
One implementation of a RAID 3 architecture is the Micropolis Corporation Parallel Drive Array, Model 1804 SCSI, that uses four parallel, synchronized disk drives and one redundant parity drive. The failure of one of the four data disk drives can be remedied by the use of the parity bits stored on the parity disk drive. Another example of a RAID 3 system is described in U.S. Pat. No. 4,092,732 to Ouchi.
A RAID 3 disk drive memory system has a much lower ratio of redundancy units to data units than a RAID 2 system. However, a RAID 3 system has the same performance limitation as a RAID 2 system, in that the individual disk actuators are coupled, operating in unison. This adversely affects the random access performance of the drive array when data files are small, since only one data file at a time can be accessed by the "single" actuator. Thus, RAID 3 systems are generally not considered to be suitable for computer systems designed for OLTP purposes.
A RAID 4 architecture uses the same parity error correction concept of the RAID 3 architecture, but improves on the performance of a RAID 3 system with respect to random reading of small files by "uncoupling" the operation of the individual disk drive actuators, and reading and writing a larger minimum amount of data (typically, a disk sector) to each disk (this is also known as block striping). A further aspect of the RAID 4 architecture is that a single storage unit is designated as the parity unit.
A limitation of a RAID 4 system is that Writing a data block on any of the independently operating data storage units also requires writing a new parity block on the parity unit. The parity information stored on the parity unit must be read and XOR'd with the old data (to "remove" the information content of the old data), and the resulting sum must then be XOR'd with the new data (to provide new parity information). Both the data and the parity records then must be rewritten to the disk drives. This process is commonly referred to as a "Read-Modify-Write" (RMW) sequence.
Thus, a Read and a Write on the single parity unit occurs each time a record is changed on any of the data storage units covered by a parity record on the parity unit. The parity unit becomes a bottle-neck to data writing operations since the number of changes to records which can be made per unit of time is a function of the access rate of the parity unit, as opposed to the faster access rate provided by concurrent operation of the multiple data storage units. Because of this limitation, a RAID 4 system is generally not considered to be suitable for computer systems designed for OLTP purposes. Indeed, it appears that a RAID 4 system has not been implemented for any commercial purpose.
A RAID 5 architecture uses the same parity error correction concept of the RAID 4 architecture and independent actuators, but improves on the writing performance of a RAID 4 system by distributing the data and parity information across all of the available disk drives. Typically, "N+1" storage units in a set (also known as a "redundancy group") are divided into a plurality of equally sized address areas referred to as blocks. Each storage unit generally contains the same number of blocks. Blocks from each storage unit in a redundancy group having the same unit address ranges are referred to as "stripes". Each stripe has N blocks of data, plus one parity block on one storage device containing parity for the N data blocks of the stripe. Further stripes each have a parity block, the parity blocks being distributed on different storage units. Parity updating activity associated with every modification of data in a redundancy group is therefore distributed over the different storage units. No single unit is burdened with all of the parity update activity.
For example, in a RAID 5 system comprising 5 disk drives, the parity information for the first stripe of blocks may be Written to the fifth drive; the parity information for the second stripe of blocks may be Written to the fourth drive; the parity information for the third stripe of blocks may be Written to the third drive; etc. The parity block for succeeding stripes typically "precesses" around the disk drives in a helical pattern (although other patterns may be used).
Thus, no single disk drive is used for storing the parity information, and the bottle-neck of the RAID 4 architecture is eliminated. An example of a RAID 5 system is described in U.S. Pat. No. 4,914,656 to Clark et al.
As in a RAID 4 system, a limitation of a RAID 5 system is that a change in a data block requires a Read-Modify-Write sequence comprising two Read and two Write operations: an old parity (OP) block and old data (OD) block must be read and XOR'd, and the resulting sum must then be XOR'd with the new data. Both the data and the parity blocks then must be rewritten to the disk drives. While the two Read operations may be done in parallel, as can the two Write operations, modification of a block of data in a RAID 4 or a RAID 5 system still takes substantially longer then the same operation on a conventional disk. A conventional disk does not require the preliminary Read operation, and thus does not have to wait for the disk drives to rotate back to the previous position in order to perform the Write operation. The rotational latency time alone can amount to about 50% of the time required for a typical data modification operation in a RAID 5 system. Further, two disk storage units are involved for the duration of each data modification operation, limiting the throughput of the system as a whole.
FIG. 1 is block diagram of a generalized RAID 4 system in accordance with the prior art. Shown is a Central Processing Unit (CPU) 1 coupled by a bus 2 to an array controller 3. The array controller 3 is coupled in a RAID 4 configuration to each of the plurality of failure-independent storage units S1-S4 (four being shown by way of example only) and a parity storage unit 4 by an I/O bus 5 (e.g., a SCSI bus).
FIG. 2 shows a high-level flow chart of the steps which must be taken to write a new data (ND) block onto one storage unit of a redundancy array of the type shown in FIG. 1. A typical RMW sequence begins by reading the OD block which will be rewritten by the ND block from one of the four storage units S1-S4 (step 200). The OD block is then transmitted from the storage unit to a controller (step 201). The corresponding old parity (OP) block must then be read from the parity storage unit (step 202) and transmitted to the controller (step 203). Once the OD block and the OP block are present in the controller, they are XOR'd to remove the information content of the OD block from the OP block. The ND block is XOR'd with the XOR sum of the OD block and the OP block to create a new parity (NP) block (step 204). The NP block is then transmitted to (step 205) and Written to (step 206) the parity storage unit. The ND block is then transmitted to (step 207) and Written to (step 208) the storage unit from which the OD block was Read and thereby overwrites the OD block.
The entire RMW sequence requires a total of two Read operations, two Write operations, two XOR operations, and four transmissions between the storage units and the controller.
Due in large part to the amount of time required to initiate and complete a transmission of a block of data between a storage unit and the controller, the RMW sequence takes longer than is desirable. Additionally, even when the two Read operations are done in parallel and the two Write operations are done in parallel, both the storage unit which holds the data and the storage unit which holds the parity information are unavailable for subsequent RMW sequences which could otherwise be started concurrent with a portion of the previous RMW sequence.
For example, in a RAID 5 configuration, assume that one record to be modified is stored in S1 and the associated parity information is stored in S2. A second record which is to be modified is stored in S2 and the associated parity information is stored in S3. Because S2 must be accessed during the modification of the record stored in S1, the present art does not teach how to begin a parallel RMW operation to modify the data stored in S2 until completion of the RMW operation being performed on the data in S1.
The most efficient way to utilize the storage units is to allow each unit to be accessed as soon as it is free to reduce the sum of the time that both storage units involved in a particular RMW operation are unavailable for other RMW operations.
It is therefore desirable to reduce the number of operations, and particularly the number of transmissions between storage units and the controller, which must be performed in the RMW sequence. The present invention provides such a method.